1. Field of the Invention
The subject matter of the invention is a method for the continuous preparation of alkali alcoholates by the transalcoholization of alkali alcoholates of lower alcohols, especially of methanol or ethanol, with higher alcohols provided in excess, especially aliphatic or cycloaliphatic univalent alcohols, and the removal of the lower-boiling alcohol.
2. Discussion of the Prior Art
It is known to prepare sodium methylate by passing an aqueous, preferably 50 weight-percent sodium hydroxide solution countercurrently to methanol vapor thereby causing them to react. This method, however, is very complex, especially when it is used in the preparation of alkali alcoholates which are derived from alcohols of 2 or more carbon atoms. In order to obtain sufficiently pure alcoholates and a high transformation, the alcohol that is charged must be substantially anhydrous. 77 to 84% of the alcohol input passes out through the top of the reactor and contains all of the water. The separation of this mixture into alcohol for recycling to the reaction and into alcoholfree water to comply with environmental protection requirements is very costly in terms of apparatus and energy.
It is also known to prepare alkali metal alcoholate by reacting 100% sodium methylate with the alcohol of the desired alcoholate in a stirring tank, the methanol that forms being removed from the system. Since this reaction is a pure equilibrium reaction of the general formula: EQU MeOR+R'OH.revreaction.MeOR'+ROH
Me=alkali metal PA1 R=lower alcohol PA1 R'=higher alcohol
It becomes very difficult to prepare high-purity alkali metal alcoholates while achieving a high alkali metal methylate transformation because of the need for a thorough removal from the system of the methanol that forms. Moreover, the separation of the alcohol mixture taken from the stirrer-equipped reactor is expensive in terms of technology and energy.
In another known process (DT-PS No. 12 54 612), a solution of sodium methylate in excess amounts of a transalcoholization alcohol is fed countercurrently to the vaporized transalcoholization alcohol in a column. The desired new alcoholate solution is withdrawn at the bottom, and the methanol that forms is removed with the excess amount of transalcoholization alcohol at the top and this mixture is separated in a second column. For the achievement of an extensive sodium methylate teransformation, a relatively great amount of transalcoholization vapor is used for the purpose of removing the methanol from the equilibrium. This, too, requires a great amount of energy both for the performance of the transalcoholization itself and for the subsequent separation of the methanol and transalcoholization alcohol. This greatly detracts from the enconomy of the process. Furthermore, the amount of apparatus and hence the investment cost required for the production of a pure alcoholate is likewise great.
Also, the adjustment and the maintenance of a constant concentration of the alcoholate in the reaction product, which is specific in each individual case, is difficult. By means of the heating system provided in the bottom of the column, this concentration is adjusted in relation to the solubility behavior of the end product, to the quantity and the temperature of the higher alcohol vapor fed to the bottom of the column, and to the amount of vapor withdrawn at the top of the column. Even slight uncontrolled differences in the heat input produce fluctuations of the concentration, which can give rise to trouble. If the alcoholate concentration in the alcoholate solution being removed is too high, there is the danger that, even in the event of a slight temperature increase in the jacket-heated pipelines through which the solution of the alcoholate of the higher alcohol is taken from the column, an evaporation of alcohol can occur. This can result in a pecipitation of solid matter. A slight lowering of the temperature in the heated pipelines can also cause a precipitation of solid matter when the alcoholate concentration is too high, and this can cause great trouble by clogging the lines.
Too low an alcoholate concentration in the end product, on the other hand, necessitates a greater investment in apparatus for the subsequent concentration of the solution to refine the alkali alcoholate or to prepare concentrated solutions of the alkali alcoholate in the higher alcohol.